Air-source heat pumps are well known in the art. See, e.g., U.S. Pat. No. 6,615,602 (Wilkinson), which describes a typical air-source heat pump in detail. Air-source heat pumps incorporate a combination of compressors and condensers in a closed-loop system to draw heat energy from the outside environment for use in heating interior spaces. They can also be used in reverse to provide for air conditioning of interior spaces. Air-source heat pumps rely on well-known principles of thermodynamics to extract energy from a given volume of air.
As fossil fuels such as oil and natural gas continue to become scarcer, increased emphasis will be placed upon the use of electricity to provide space heating of homes and commercial buildings. Concerns are especially pertinent in cold weather areas such as New England. However, heat pump systems operating in cold weather environments often experience problems with efficiency. It is important to attain the highest level of efficiency as possible, as heat pumps that are more efficient save energy and money. A major problem that most heat pump systems experience is that, as the ambient outside temperature falls, the heating capacity of the heat pump system decreases drastically. Yet it is during these times of low temperatures that heating needs increase. In order to meet these needs, systems have utilized supplemental electrical resistance type heating, cascade technology, and boosters to increase their heating capacity so as to operate in low temperature environments.
A typical air-source heat pump is arranged with a “high side” and a “low side” configuration, wherein the system refrigerant is at a relatively high pressure and high temperature on the high side and is at a relatively low pressure and low temperature on the low side. Relatively low pressure/low temperature gaseous refrigerant from the low side is introduced into a compressor, which compresses the refrigerant into a high pressure/high temperature gas (compressing a given volume of gas into a smaller volume of gas causes its pressure and temperature to increase). The compressed high pressure/high temperature gas is then forced through a condenser which is in contact with the interior space to be heated; the gas gives up some of its energy in the condenser, thus providing heat to the interior space, and the refrigerant becomes liquefied. The liquid refrigerant is then forced through an expansion device which vaporizes the liquid into a low pressure/low temperature gas. Once the refrigerant has been vaporized into a low pressure/low temperature gas, it is passed through an evaporator which is in contact with the outside air. Heat energy is absorbed from the outside air by the refrigerant, which is then introduced to the compressor, repeating the cycle. The portion of the heat pump system between the compressor and the expansion device is the system high side, and the portion of the system from the evaporator back to the compressor is the system low side.
The foregoing is a simplified explanation of the mechanics of how an air-source heat pump works. However, it is sufficient to illustrate a phenomenon of thermodynamics which renders the typical air-source heat pump inefficient in cold climates. The maximum energy that can be extracted from a given volume of air by an air-source heat pump is its heating capacity. The heating capacity of an air-source heat pump changes with the temperature of the air from which energy is extracted. As the temperature of the outside air decreases, the expansion device pressurizes less of the refrigerant, resulting in the refrigerant having a lower density (and pressure) for a given volume to achieve a lower boiling point (since the boiling point of the refrigerant must be lower than the temperature of the ambient outside air). As the mass density of the refrigerant decreases eventually the flow of refrigerant will be below the operating capacity of the heat pump. Because air-source heat pumps are designed to handle a specific volume of flow, lowering the amount available lowers the overall heating capacity of the heat pump, because the system high side requires the refrigerant to be of a certain minimum pressure; when the refrigerant pressure is diminished due to decreasing outside air temperatures the compressor must raise the pressure of the refrigerant a greater degree. When the outside temperature becomes cold enough the corresponding pressure differential between the system low side and the system high side becomes too great for the compressor to overcome. To compensate, either compressors with far greater maximum capacity must be used, at great expense and inefficiency, or alternative heat sources must be available when the outside temperature falls too low. Neither of these solutions is practical and thus the use of typical air-source heat pumps is very limited in colder climates, where the need for heat is greatest during those winter months when the outside air is coldest and the resulting heating capacity is lowest.
A solution to the lack of efficiency of cold climate air-source heat pumps was demonstrated by Gustafsson, involving the use of cascade-connected heat pumps. See, e.g., U.S. Pat. No. 3,984,050 (Gustafsson). Cascading heat pumps are well known in the art. Gustafsson describes a heat pump system capable of extracting heat from relatively low temperature ambient air (−10° C.) to produce hot water (up to 80° C.). Air-source heat pump systems set up in a cascade fashion use the condenser unit of one heat pump arranged in a heat-exchanging relationship with the evaporator of the other heat pump. This “piggy-back” relationship increases the system's efficiency, i.e., the ratio between the output energy and the input energy. However, while the Gustafsson device and similar systems are able to reach temperatures high enough to produce hot water, they do not also provide air conditioning.
A cascading heat pump system capable of operating over a wide range of source temperature and of providing supplemental comfort zone air conditioning was disclosed in U.S. Pat. No. 4,391,104 (Wendschlag). The Wendschlag heat pump system uses a first refrigerant fluid and a second refrigerant fluid with separate compression cycle loops passing in heat transfer relationship through a tri-fluid heat exchanger. Having separate circuits allows several different types of refrigerant to be used, which is beneficial because different types of refrigerant are effective under different conditions. This setup also allows the system to operate in several different modes, incorporating a method for selectively heating water by extracting heat from relatively cold outdoor ambient air in cascade, and for heating or cooling air supplied to a comfort zone in non-cascade fashion. Although the Wendschlag system, and others like it, work efficiently in cold weather, they are not effective when it comes to providing air conditioning during periods of warm temperatures. The air conditioning provided by Wendschlag is only supplemental and occurs only as a by-product of heating water. This prevents Wendschlag and similar systems from being efficient sources of comfort zone temperature conditioning, and necessitate that a separate air conditioning system be used in order to provide efficient and sufficient cold air during warm weather months.
One solution to the problem of cold climate heat pumps which also efficiently provide air conditioning was demonstrated in U.S. Pat. No. 4,149,389 (Hayes, et al.). Hayes, et al., uses either a cascading or non-cascading mode to send hot air into the conditioned space. The non-cascade heating mode also operates in reverse in order to provide non-supplemental air conditioning to the conditioned space. However, the Hayes, et al., device is not set up to run an external heating system and a separate external cooling system. Rather, the output heat energy is exhausted through an air plenum into the ambient interior air, which also serves as the location for the provision of air conditioning. The Hayes, et al., system is therefore impractical for application with existing heating systems or where more efficient heating systems are desired.
It is therefore an object of the invention to provide an improved air-source heat pump which operates efficiently in cold climates.
It is a further object of the invention to provide an improved air-source heat pump which incorporates the efficiencies of a cascading dual refrigerant circuit heating system.
It is yet a further object of the invention to provide an improved air-source heat pump which incorporates the efficiencies of a cascading heating system with a non-cascading cooling system.
It is yet a further object of the invention to provide an improved air-source heat pump which incorporates different refrigerants to increase the operational temperature range of the device.
It is yet a further object of the invention to provide an improved air-source heat pump which efficiently integrates with a furnace or a boiler.
It is yet a further object of the invention to provide an improved air-source heat pump which efficiently integrates directly with a hot water line or a hot air line.
It is yet a further object of the invention to provide an improved air-source heat pump which efficiently integrates directly with a cold water line or a cold air line.
It is yet a further object of the invention to provide an improved air-source heat pump wherein the components of a cascading dual refrigerant circuit are simplified resulting in the overall efficiency of the system being maximized and the time and effort for installation being minimized.
Other objects of this invention will be apparent to those skilled in the art from the description and claims which follow.